The present invention relates generally to pull-up riser tensioner systems used on offshore floating production and drilling platforms and, more particularly, to a pull-up riser tensioner system with a fluid retention device.
The current technology for cylinders used on riser tensioners is illustrated in FIG. 1. A typical prior art pull-up riser tensioner system 100 includes multiple hydro-pneumatic cylinders 102. The lower ends of the cylinders are attached to a threaded tension ring 104 that is connected to a production or drilling riser 106. The opposite, top ends of the cylinders are connected to the platform structure 108. The cylinders 102 serve to maintain a near constant tension on the production or drilling riser 106 as the platform 108 moves in multiple planes due to wind, waves, platform movements, and other natural events.
A typical prior art cylinder accumulator system 200 is illustrated in FIG. 2. The cylinder accumulator system 200 is comprised of a hydraulic cylinder 202, high-pressure accumulator 204, low-pressure accumulator 206, and various pipes 208 between the accumulators 204, 206 and cylinder 202. The hydraulic cylinder 202 comprises a cylinder piston assembly 210, a cylinder rod 212, cylinder end cap 213, a cylinder head 214, and a cylinder rod extension 215. The high-pressure accumulator 204 serves as a gas “spring.” Under normal operation, the high-pressure accumulator 204 holds both hydraulic fluid and a gas media such as nitrogen or compressed air. The size or volume of the high-pressure accumulator 204, relative to the cylinder volume, determines the overall spring rate or stiffness of the cylinder accumulator system 200 as the cylinder accumulator system 200 moves over its stroke range. The low-pressure accumulator 206 serves to maintain a near constant pressure on the piston side of the hydraulic cylinder 202 over the stroke range and maintains a closed system.
Current cylinder technology requires a large volume of hydraulic fluid that is equal to at least 115% of the total cylinder swept volume. The volume of hydraulic fluid is based on the total swept volume plus an additional 15% reserve that is used to account for any small leaks that may occur while in operation. Maintaining this volume is required in order to maintain fluid contact with the dynamic seals that are used on both the cylinder piston assembly and the cylinder rod. Under extreme weather events and vessel offsets, the riser tensioner cylinders can be stroked into the fully extended position which pushes nearly all of the hydraulic fluid into the high-pressure accumulator. The high-pressure accumulator is sized to hold this volume plus the additional gas volume in order to maintain the desired spring constant or stiffness. Depending on the riser stiffness requirements, the high-pressure accumulators can be very large and can present problems with weight, packaging, and cost.
Hydraulic fluid used in tensioners contains additives to improve its performance and add characteristics to the hydraulic fluid, such as corrosion protection. Over time, the hydraulic fluid typically breaks down and must be replaced while the tensioner system is still in service supporting the riser weight. Current cylinder technology requires disconnecting the cylinder from the tension ring, fully extending the rod to force all the fluid out and refilling the cylinder with fluid. This can be a time-consuming process and care must be taken to ensure that the cylinder side of the system is full of fluid with no gas bubbles. Then the cylinder is re-attached to the tension ring and pressured up before the process is repeated for the remaining cylinders. Connecting and disconnecting a cylinder from the tension ring can be a time-consuming process, particularly given that access to the tension ring is limited on many platform structures.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.